In some internal combustion engines, a passage for allowing blowby gas to flow from the crank chamber to the valve operating chamber and a passage for allowing oil to flow from the valve operating chamber to the crank chamber are formed separately from each other in the cylinder block and the cylinder head, such that interference between the blowby gas and the oil is avoided and each of the blowby gas and the oil can flow smoothly (see JP2009-180172A, for example).
The blowby gas is supplied to a gas-liquid separator to remove liquid therefrom before being supplied to the intake system of the internal combustion engine. In the gas-liquid separator, liquid oil that has been removed from the blowby gas is produced, and therefore, a passage for returning this oil to the oil pan at a lower part of the crank chamber is necessary. Thus, such internal combustion engines need to be provided with a blowby gas passage and an oil return passage, and it is necessary to arrange them efficiently. However, it is desired to prevent contact between the blowby gas and the oil returned to the oil pan as much as possible, to minimize entrance of oil droplets into the blowby gas.